As a conventional hydraulic drive system for construction machines such as hydraulic excavators, there is known a load sensing system for controlling a delivery rate of a hydraulic pump so that a delivery pressure of the hydraulic pump is held higher a fixed value than a maximum load pressure among a plurality of actuators. Generally, this system includes a plurality of flow control valves for controlling respective flow rates of a hydraulic fluid supplied from the hydraulic pump to the plurality of actuators, and pressure compensating valves, called distribution compensating valves, arranged upstream of the respective flow control valves for controlling differential pressures across the flow control valves. With the provision of the distribution compensating valves, when plural actuators are simultaneously driven in the combined operation, the hydraulic fluid is surely supplied to the actuator on the lower load side as well for the smooth combined operation.
W090/00683 (corresponding to U.S. Pat. No. 5,056,312) discloses one developed form of such a load sensing system. The disclosed system comprises a differential pressure sensor for detecting a differential pressure between the pump delivery pressure and the maximum load pressure, i.e., an LS differential pressure, and outputting a corresponding differential pressure signal, a memory for storing a plurality of data patterns which are associated with types of the actuators and used to individually compute set values of the distribution compensating valves, and a computing control unit for computing the set values dependent upon the differential pressure signal from the plurality of data patterns. In the combined operation in which plural actuators are simultaneously driven, by individually controlling the set values of the distribution compensating valves based on the above computed values, the hydraulic fluid can be not only supplied to the actuator on the lower load side as well, but also supplied to the actuators at distribution ratios suitable for their types, thereby improving operability even under a saturated condition in which the delivery rate of the hydraulic pump is insufficient.
In the above system, each of the distribution compensating valves comprises a first pressure bearing chamber subjected to a pressure upstream of the associated flow control valve for acting in a valve-closing direction, a second pressure bearing chamber subjected to a pressure downstream of the associated flow control valve for acting in a valve-opening direction, means for applying a certain control force in the valve-opening direction to set a target value of the differential pressure across the associated flow control valve, and a third pressure bearing chamber subjected to a control pressure from a solenoid proportional control valve for acting in the valve-closing direction to reduce the above differential pressure target value. The computing control unit computes a target reducing value for the differential pressure target value and outputs a corresponding signal to the solenoid proportional control valve which in turns produces the control pressure for a reduction of the differential pressure target value in an individual manner.
The above means for setting the differential pressure target value is usually a spring as shown in FIG. 1 of W090/00683. Also, instead of the spring, a pressure bearing chamber subjected to a certain pilot pressure is provide in FIG. 15 of W090/00683. Further, in FIG. 17 of W090/00683, the above third pressure bearing chamber acting in the valve-closing direction is omitted, and a pressure bearing chamber acting in the valve-opening direction is provided instead which can double as the third pressure bearing chamber. A control pressure introduced to that pressure bearing chamber is controlled so that the chamber may carry out both a function of the means for setting the differential pressure target value and a function of the third pressure bearing chamber.